Why are 5G QoS parameters required?
As the so-called “Internet of the Everything” network platform, the 5G network needs to carry a variety of 5G QoS parameters with a wide range of service characteristics.
These traffic with diverse service characteristics share the “limited” resources of the cellular system, which is “limited” because the radio spectrum and transmission loop resources are limited and expensive, so corresponding resource allocation rules need to be developed to optimize the allocation of resources. Therefore, corresponding resource allocation rules are needed to optimize the allocation of resources.
For example, it is necessary to contract different 5G QoS parameters, including the corresponding rate, delay, priority, packet loss rate, etc., and also to define the coping mechanism for the congestion that occurs in the network.
According to the requirements of 5G NR Rel-15 Phase 2: “The UE classifies and marks the uplink user-plane services based on QoS rules, i.e. the association of uplink services with QoS flows.” These rules can be signaled explicitly on the N1 (on PDU session establishment or QoS flow establishment), pre-configured in the UE, or derived implicitly by the UE from the reflective QoS.
“QoS flows are the best granularity for QoS differences in PDU sessions” and “every PDU session requires default QoS rules” from the network to the UE, the protocol needs to be able to provide one or more QoS rules associated with a PDU session.
What are the 5G QoS parameters?
Phase 2 requirements indicate the need to provide the following 5G QoS parameters rules.
- the 5G QoS parameters rule contains the QoS rule identifier, the QFI of the 5G QoS parameters flow, and the 5G QoS parameters flow template (i.e.: the set of packet filters and corresponding priority values associated with the 5G QoS parameters flow).
- In the UL, the UE classifies the packet data based on the 5G QoS parameters flow template in the 5G QoS parameters rule and transmits the classification of the user-plane services belonging to the 5G QoS parameters flow by using the user-plane tokens of the QFIs in the corresponding 5G QoS parameters rule.
- for UEs that support the reflected 5G QoS parameters feature and if the 5GC enables the reflected 5G QoS parameters to feature for some service flows, the UE shall create derived 5G QoS parameters rules for the uplink services based on the received downlink services. the UE shall use the derived 5G QoS parameters rules to determine the mapping between the uplink services and the 5G QoS parameters flows.
- when reflective QoS is activated via the control plane, the priority value of the derived QoS rules (i.e. QoS flows, PDU sessions) within the control plane activation range is set to the value signaled via the control plane.
That is, each QoS rule includes at least:
QoS rule identifier
QFI
QoS flow template containing a set of packet filters and the corresponding priority values
Whether to use the control plane to activate the reflected QoS feature
The priority value of the derived QoS rule if the control plane is used to activate the reflective QoS functionality.
Phase 2 also defines the following 5G QoS parameters related to QoS flows.
5G QoS Indicator (5QI).
Allocation and Retention Priority (ARP: Allocation and Retention Priority).
Guaranteed Flow Bit Rate (GFBR: Guaranteed Flow Bit Rate), UL, and DL (for GBR QoS flows only).
Maximum Flow Bit Rate (MFBR: Maximum Flow Bit Rate), UL, and DL (GBR QoS flows only).
Notification Control (GBR QoS flows only).
This facilitates the provision of guaranteed traffic for the uplink (GFBR), guaranteed traffic for the downlink (GFBR), maximum traffic for the uplink (GFBR), and maximum traffic for the downlink to the UE (GFBR) since similar rates to the UE are provided in the EPS NAS and used by the upper layers of the UE (e.g. IMS).
It is beneficial to provide the 5G QoS Indicator (5QI) to the UE as the semantics of 5QI, similar to the semantics of the QCI of the EPS: a scalar used as a reference to a specific packet forwarding behavior (e.g. packet loss rate).
The EPS NAS does not provide allocation and reservation priority and notification control to the UE, so it does not seem advantageous to provide these priority and notification controls to the upper layers via the 5GS NAS.
The length of the 5QI is not specified in the second phase. Assuming that the semantics of 5QI is similar to that of QCI in EPS and assuming that the standard code point assignments in TS 23.501 and TS 23.203 are similar, the length of 5QI should be the same as the length of 5QI, i.e. 8 bits.
The length of the 5QI affects the length of the QFI because phase 2 states that “For non-GBR QoS flows with normalized 5QI, the normalized 5QI value is used as the QFI, and the default ARP is used.” so the QFI should be at least as long as the 5QI, i.e., at least 8 bit long.
Furthermore, assuming that “the CN transmits the classification of the user-plane services belonging to the QoS flow by using the N3 (and N9) user-plane tags of the QFI” and “the necessary AN resources are established by the access network to map the QoS flow to the DRB for the UE to receive the QFI (and reflective QoS can be applied) “, QFI is also used for the access layer, N2 reference point, and N3 reference point.
According to the requirements of Phase 2: “For non-GBR services using session AMBR, the UE shall enforce UL rate limits on a PDU session basis if the UE receives a session AMBR”, the protocol needs to allow the provision of session AMBR from the network to the UE.